Organic semiconductor materials are one of the most important materials in the field of organic electronics, and can be classified into electron-donating p-type organic semiconductor materials and electron-accepting n-type organic semiconductor materials. Various semiconductor elements can be produced by appropriately combining p-type organic semiconductor materials and n-type organic semiconductor materials, and these elements are applied to, for example, organic electroluminescences which emit light under the action of excitons formed by recombination of electrons and holes, organic thin-film solar cells which convert light into electric power, and organic thin-film transistors which control an amperage and a voltage.
Among them, organic thin-film solar cells are useful for environmental conservation because they do not release carbon dioxide into the air, and also, organic thin-film solar cells are easily produced because they have a simple structure. Therefore, the demand of organic thin-film solar cells is increasing. However, the photoelectric conversion efficiency of the organic thin-film solar cell is not sufficient yet. The photoelectric conversion efficiency η is a value calculated as a product of a short-circuit current density (Jsc), an open circuit voltage (Voc) and a fill factor (FF) (η=open circuit voltage (Voc)× short circuit current density (Jsc)× fill factor (FF)), and for improving the photoelectric conversion efficiency, it is necessary to improve the short-circuit current density (Jsc) and the fill factor (FF) as well as the open circuit voltage (Voc).
The open circuit voltage (Voc) is proportional to a difference in energy between the HOMO (highest occupied molecular orbital) level of a p-type organic semiconductor and the LUMO (lowest unoccupied molecular orbital) level of a n-type organic semiconductor, and therefore for improving the open circuit voltage (Voc), it is necessary to deepen (lower) the HOMO level of the p-type organic semiconductor.
The short-circuit current density (Jsc) correlates to the amount of energy received by an organic semiconductor material, and for improving the short-circuit current density (Jsc) of the organic semiconductor material, it is necessary for the organic semiconductor material to absorb light in a wide wavelength range extending from a visible region to a near-infrared region. The wavelength of a light having the lowest energy in the light that can be absorbed by the organic semiconductor material (the longest wavelength) is an absorption edge wavelength, and the energy corresponding to this wavelength is equal to band gap energy. Accordingly, for the organic semiconductor material to absorb light in a wider wavelength range, it is necessary to narrow the band gap (difference in energy between the HOMO level and the LUMO level of the p-type organic semiconductor).
On the other hand, in Patent Document 1, a compound having a benzobisthiazole backbone is proposed, but conversion efficiency is not known.